Carbon graphite has self-lubricating property, high mechanical strength, low friction coefficient, high temperature resistance and good abrasion resistance. It is widely used in machinery, chemical industry, petroleum, instruments and water pumps. Conventional graphite materials are not ideal for abrasion resistance and corrosion resistance, while silicified graphite, as a novel engineering material, has the properties of carbon graphite and silicon carbide, and is widely used.
1. What is silicified graphite?
Silicified graphite is a composite material formed by coating the surface of graphite with silicon carbide. The thickness of silicon carbide layer is 1 ~ 1.5mm. The hardness of silicified graphite is SiC, second only to diamond, boron nitride and boron carbide, and higher than tungsten carbide and aluminum trioxide.
Silicified graphite not only has the carbon graphite material self lubrication, conductive thermal conductivity, good thermal shock resistance and impermeability, but also has high strength of silicon carbide, oxidation resistance, resistance to chemical corrosion, etc, and especially suitable for applications in harsh situation of overload and high temperature, and therefore, silicified graphite material widely used in sealing, friction, chemical, metallurgy, aerospace and nuclear industry and other fields.
2. Preparation of silicified graphite
The production methods of silicified graphite mainly include three chemical vapor deposition methods (CVD), chemical vapor reaction (CVR) and liquid silica osmosis reaction.
Chemical vapor deposition (CVD) methodThe SiC was deposited on the surface of the graphite substrate by pyrolysis of the gas containing silicon and carbon through the high temperature graphite substrate. The raw materials are trichloromethyl silane (CH3SiC3), silicon tetrachloride, hydrogen, silicon vapor, etc. Deposition temperature range is wide, ranging from 1175 ℃ to 1775 ℃. The SiC layer generated by this method is very dense, with uniform thickness and general thickness of 0.1 ~ 0.3mm. However, the combination of SiC and graphite substrate is purely mechanical, and the binding force is weak. In case of temperature change, the SiC layer is prone to crack and flake.
Chemical vapor reaction (CVR)
Raw material for coke powder and quartz sand or amorphous silicon powder, when heated to 2000 ℃ produces chemical reaction, generate the SiO steam. SiO vapor and carbon matrix react to form SiC. There is no obvious interface between the SiC layer and the carbon matrix, and the bond is very strong, and it will not fall off in the case of temperature change and high load. However, the CVR method is that SiO gas infiltrates into the carbon matrix for reaction, so the porosity of the carbon matrix is still retained.
Liquid silicon penetration method
This method is also a kind of CVR. Under the condition of vacuum, heated to 1700-1900 ℃, the carbon matrix directly into molten silicon liquid, liquid silicon gradually penetrate into internal carbon matrix, and react to generate the SiC. The raw material is 99.9999% pure silicon. SiC layer thickness up to 3.5mm. After the reaction, the carbon matrix contains about 17% free silicon filled in the pores of the matrix, making the matrix dense and impermeable. But the existence of free silicon reduces the corrosion resistance and oxidation resistance of silicified graphite at high temperature.